Method and apparatus for the real-time characterization of particles suspended within a fluid medium

ABSTRACT

This invention describes a method by which microparticles, typically in the size range from 0.3 μm to 100 μm, which are carried in a fluid suspension, may be rapidly detected and characterized. The method primarily relates to the measurement of atmospheric particles such as those in clouds or environmental aerosols, but it may be used to measure other forms of particulate suspension wherever the flow of suspension through a defined measurement space can be achieved. The method is based upon a rapid analysis of the spatial laser scattering profile (i.e., the complex manner in which individual particles scatter laser light) recorded from individual particles as they are carried in suspension through a measurement space. Using this method it is possible to differentiate various types of particles based on particle shape and structure, as manifest in characteristics of their individual spatial light scattering patterns. The sizes of spherical particles and the spherical equivalent sizes of non-spherical particles may also be determined, allowing size distribution for each particle type within the suspension to be determined. An implementation of the method for use in an aircraft mounted instrument is described.

FIELD OF THE INVENTION

The present invention relates to apparatus for and methods ofclassifying particle shape in a fluid. It is particularly applicable,but in no way limited to, the real-time classification of particle shapein the atmosphere.

BACKGROUND TO THE INVENTION

In investigations of the composition and dynamics of the earth'sparticulate atmosphere, particle shape is an important parameter bywhich classification and possibly identification of particles may oftenbe achieved. Spherical droplets, cuboidal crystals typical of marineaerosols, and the wide variety of morphologies assumed by ice crystals,are examples where the determination of shape may be used in combinationwith size spectra measurements to provide experimental data upon whichtheoretical models of macroscopic and microscopic physical behaviour ofclouds and aerosols may be developed and tested. A specific example ofthis involves the study of ice microphysics and the behaviour ofdroplets and ice crystals which can occur simultaneously within clouds.The radiative properties of these mixed-phase clouds can be radicallydependent upon the relative proportions and size spectra of the twophases, as well as the orientations of the ice crystals present, andthis has a profound effect upon the proportion of incident sunlightreaching the lower atmosphere and earth surface. To be able tounderstand the radiative transfer properties of ice and mixed phaseclouds, a detailed knowledge of the particles' shapes and sizes isrequired, along with measurements of the number concentration of ice andsuper-cooled liquid water particles. Furthermore, measurement of thetotal ice crystal number is important to facilitate the testing oftheories of the nucleation of ice crystals and their role in climatechange.

Existing Atmospheric Particle Measurement Techniques

Whilst there are several commercially available aircraft-mountedinstruments designed to measure the size spectra of atmosphericparticles. The FSSP—Forward Scatter Spectrometer Probe, from ParticleMeasurement Systems Inc. Boulder Colo.—is perhaps the most widely usedin airborne platforms. These instruments cannot provide informationrelating to particle shape. They are generally calibrated on theassumption that all particles are spherical and they are thus incapableof discriminating between, for example, ice crystals and water dropletsof equivalent optical scattering size.

In regard to particle shape measurement, the 2D-OAP-2D—Optical ArrayProbe from Particle Measurement Systems Inc.—is commonly employed forexamining airborne particles greater than ˜30 μm in size. Thisinstrument records a silhouette of individual particles as they passthrough a light sheet and occlude elements within a linear detectorarray arranged orthogonally to the particle trajectory. However, the2D-OAP suffers a number of limitations, principally:

(i) it provides only limited instantaneous information of the range ordistribution of particle sizes within a measured atmosphere(post-processing in the laboratory is normally undertaken to assessshape spectra);

(ii) it suffers from a number of artefacts produced in the recorded databy events such as the collection and subsequent release of liquid waterdrips from the leading points of the probe arms or the ‘splashing’ oflarge droplets on these arms producing artificially high populations ofsmall droplets; and most importantly;

(iii) it cannot accurately resolve the shapes of particles which occludeless than the order of five array pixels, corresponding to particlesizes below about 125 μm [Moss S. J. and Johnson D. W. AtmosphericResearch 34 pp. 1-25, 1994]. The inability to analyze and categorize theshapes of smaller particles makes it impossible for the instrument todifferentiate water droplets from ice crystals for these sizes.Therefore the instrument is unable to provide data which can answer themicrophysical questions concerning the radiative transfer properties of,for example, cirrus clouds in which the ice particle and water dropletsizes are frequently well below the limit of resolution of the 2D-OAP.

The shapes of smaller particles, theoretically down to ˜2-4 μm but inpractice down to ˜10 μm because of aircraft vibration, can be assessedusing a holographic technique [Brown, P. R. A. j. Atmos. OceanicTechnol., vol. 6, pp. 293-306, 1989]. This technique involves using apulsed Nd:YAG laser and photographic film recording system to acquireholographic ‘snapshots’ of particle populations within a measurementspace. The processed holograms are later interrogated using a CW laserto recreate the images of the particles, allowing detailed analysis.This process is extremely slow and manually intensive, taking up to aday for each hologram, and again, the smaller particles of interest arebeyond the instrument's limit of resolution.

Spatial Light Scattering Techniques

The applicants have developed several ground-based instruments for theclassification and identification of airborne particles by analysis ofthe manner in which individual particles spatially scatter incidentlaser illumination. These are described in ‘Portable ParticleAnalysers’. Ludlow, I. K. and Kaye P. H. European Patent EP 0 316 172,July 1992; ‘Particle Asymmetry Analyser’. Ludlow, I. K. and Kaye, P. H.European Patent EP 0316 171, September 1992 which represent the closestprior art known to the applicant. In these instruments, airborneparticles are drawn from the ambient atmosphere by a suction pump andare constrained by narrow delivery tubes, typically 1 mm in diameter,and a surrounding layer of filtered sheath air, to pass through anincident laser beam within a laser scattering chamber. The intersectionof the particle flow with the beam defines the measurement space throughwhich all particles in the sample flow will pass. Particle flow is suchthat statistically, particle coincidences within the measurement spaceare rare. Each particle passing through the measurement space willscatter light in a manner which is governed inter alia by the size,shape, and structure of the particle. FIG. 1 shows typical lightscattering patterns recorded from individual microscopic airborneparticles. The black circle at the centre of each pattern is caused by abeam stop, and the outer circumference of the patterns corresponds toscattering at an angle of approximately 35° to the direction of theincident beam. As can be seen in FIG. 1, spherical particles such asdroplets produce a regular concentric ring scattering patterns, whilstelongated particles such as fibres or long crystals produce linearscattering angled according to the orientation of the particle.Irregular shaped particles may produce more complex patterns with feweasily discernible features. In the instruments described in theaforementioned prior art, the scattering patterns as shown in FIG. 1 arecollected by the three detectors arranged symmetrically about the laserbeam axis. By measurement of the difference in magnitude of the signalsreceived from the three detectors, a crude estimate of the shape of thescattering particle may be deduced. However, the type of instrumentdescribed above is not suitable for measuring atmospheric particles suchas ice crystals or super-cooled water droplets for the following reason:in the measurement of atmospheric particles it is essential that neitherthe phase (ie: ice or water) nor the orientation (which governsradiative behaviour) of the particles is affected by the measurementprocess. This precludes the use of a pumped sample delivery system inwhich the particles are drawn from the atmosphere via narrow tubes intoa measurement chamber. Such a pumped system would certainly change theorientation of the particles and would be likely to melt or partiallymelt smaller ice crystals present in the sample.

With the foregoing argument in mind, the present invention has theobjective of providing a means by which the sizes, shapes, andorientations of fluid-borne particles may be determined rapidly andnon-intrusively, ie: in a way which will not materially affect theparticles under examination. The present invention thus providesimproved apparatus and methods for the classification andcharacterization of the shape of such particles which overcome ormitigate some or all of the above disadvantages.

SUMMARY OF THE INVENTION

According to a first aspect of the present invention there is provided aparticle analyzer for use in characterizing the shape of particles in afluid medium, said particle analyser comprising:

(i) means for providing a sample of fluid in the form of a flow througha scattering chamber;

(ii) means for generating a first beam of radiation having a firstwavelength;

(iii) means for generating a second beam of radiation having a secondwavelength wherein the first and second wavelengths are different, saidfirst and second beams of radiation being adapted to intersect eachother to create a so-called virtual measurement space;

(iv) a first detection means adapted to detect radiation scattered by aparticle passing through the first beam of radiation;

(iv) a second detection means adapted to detect radiation scattered by aparticle passing through the second beam of radiation;

(vi) means for deriving data from the radiation detected by therespective detection means;

(vii) means for comparing said derived data with data from particles ofknown shape.

By providing two intersecting beams of radiation, which are preferablylaser light, this creates a “virtual” measuring space which avoids thenecessity of using narrow delivery tubes. This has the advantage thatfluids flowing through large diameter pipes can be sampled for itsparticle content without disruption of the flow.

Preferably the first beam of radiation has a cross-section which issubstantially a narrow ellipse. This presents a relatively thin sheet oflight through which the particles pass.

Preferably the second beam of radiation has a cross-section which issubstantially circular and the diameter of the second beam of radiationis less than the widest dimension of the first beam.

In a particularly preferred embodiment the first and second beamsintersect at an angle of substantially 60°.

Preferably the first detection means comprises a plurality of individualoptical detectors. A wide variety of different arrays can be used inorder to derive information regarding particle shape.

Preferably the means for deriving data from the radiation is adapted toidentify particles which pass through the virtual measurement space andis further adapted to gather and process data from the first detectionmeans specifically derived from particles which pass through the virtualmeasurement space. This has the advantage that only particles which passthrough the virtual measuring space will produce simultaneous scatteringfrom both beams. The scattering pattern derived from only thoseparticles may be collected and processed.

Preferably the first and second detection means comprise a lens system aphotodetector, and respectively a first or second optical wavelengthfilter which allows light from only the first or second beam ofradiation to reach the respective photodetectors. This enables thescattered light to be differentiated according to its source.

Preferably an aperture is incorporated in front of the or each detectorin order to restrict the field of view of the or each detector tosubstantially the virtual measurement space, enabling spurious scattersignals to be eliminated or much reduced.

According to a second aspect of the invention there is provided a methodof particle analysis including the steps of:

(a) passing a sample of fluid through a scattering chamber;

(b) passing a first beam of radiation having a first wavelength and asecond beam of radiation having a second wavelength through the chambersuch that the two beams intersect to form a virtual measurement space;

(c) identifying those particles which pass through the virtualmeasurement space;

(d) detecting and collecting radiation having a first wavelength whichis scattered by said particles with a first detection means;

(e) converting the radiation collected into electrical signals

(f) processing and analyzing the electrical signals and comparing themwith signals from data derived from particles of known shape.

The method and thus protection sought extends to the use of any versionsof the apparatus as herein described.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be further described, by way of example, withreference to the accompanying drawings in which:

FIG. 1 illustrates typical light scattering patterns from individualparticles of various geometries;

FIG. 2 shows schematically a particle analyzer according to a firstaspect of the invention;

FIG. 3 shows diagramatically light scatter intensity signals producedfrom particles passing through and adjacent the measurement space of aparticle analyzer as illustrated in FIG. 2;

FIG. 4 illustrates examples of the outputs from a six detector array forparticles of differing shapes;

FIG. 5 shows a typical sequence for the processing of detector modulelight scattering data;

FIG. 6 shows one possible configuration of a particle detector suitablefor mounting on an aircraft.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Embodiments of the present invention are now described by way of exampleonly. These examples represent the best ways of putting the inventioninto practice that are currently known to the Applicant although theyare not the only ways in which this could be achieved.

The invention is based upon the creation of a virtual measurement space,free from mechanical obstructions and through which particles carried insuspension may pass freely, as shown in FIG. 2. The measurement space 1is created by the intersection of two laser beams of differingwavelengths, preferably inside a scattering chamber. One beam isreferred to as the Primary beam 2, the other beam is referred to as theSecondary beam 3, as shown in FIG. 2. The Primary beam 2 typically has across-section which is a narrow ellipse so that it presents a thin sheetof light through which the particle pass. The Secondary beam 3 istypically of a circular cross-section and of a diameter less than thewider dimension of the Primary beam. The angle of intersection of thetwo beams is typically 60°, such that the measurement space 1 has anapproximately circular form, the plane of which is orthogonal to thedirection of the particle motion.

Whilst these cross-sections may be preferred they are certainly not theonly cross-sections that can be used. Any two beams of radiation ofdifferent wavelengths which coincide to produce the virtual measurementspace will suffice. The angle of incidence of the two beams is notcritical but 60° has been found to work well.

In this context the term scattering chamber has a very broad meaning. Itcan refer to a defined structure into which fluid flows. Alternatively,it can refer simply to a region within a pipe, tube or duct throughwhich fluid is flowing or able to flow. It therefore need not be aseparate, defined structure.

Surrounding the measurement space but at sufficient distance from it toavoid perturbation of the suspended particles is a series of opticaldetection modules, typically eight in number, and numbered 4 to 11 inFIG. 2. As will be explained, detection modules 4 and 5 are used toestablish whether a particle has a ‘valid trajectory’, ie it passedthrough the measurement space 1. Detection modules 6 to 11 are then usedto assess particle shape and orientation. Each detection modulecomprises a lens system 12, an optical wavelength filter 13, and aphotodetector 14. (For clarity, these items are shown for only onedetection module). The lens system of each detection module is arrangedsuch that the field of view of the photodetector is just sufficient toencompass the measurement space 1. This may be achieved by suitablypositioning an aperture 15 in front of the photodetector. Particles 16travel freely through a large volume surrounding the measurement space.This is an important feature of particle analyzers according to thisinvention. The particle motion relative to the detection modules may beachieved for example by the movement through the cloud or aerosol ofinterest of an aircraft on which the detection assembly is mounted.Alternatively the motion of the particles could be caused by theirsuspension within a gas moving through a large diameter pipe or ductaround which are arranged the optical detection modules.

Particles may normally pass through the laser beams at any point alongtheir exposed length and will scatter light in all directions on doingso. However, only particles which pass through the measurement space 1defined by the intersection of the two beams will produce simultaneousscattering from both beams. This is illustrated in FIG. 3 which showsthe timing of electrical pulses from any single photodetector. Thetraces show the signals resulting from two particles 16 and 17 whosetrajectories are close to the measurement space and through themeasurement space respectively. For particle 16 the scattered lightpulses produced are separated in time as shown in the timing graphs ofFIG. 3. For particle 17 the pulses are such that the pulse derived fromthe Primary beam 2 is always contained entirely within the time durationof that derived form the Secondary beam 3. This is therefore the ‘validtrajectory’ condition for a particle trajectory through the measurementspace.

Referring again to FIG. 2, the optical filter 13 incorporated into thedetection module 5 is such that it allows only light from the Secondarybeam to pass to its photodetector. The optical filters 13 in all otherdetection modules are such that they only allow light from the Primarybeam to pass to their respective photodetectors. When a particle passesthrough the measurement space 1 it scatters light in all directions andfrom both beams simultaneously. The magnitudes of the resulting signalsfrom all detection modules are recorded instantaneously usingconventional electronic cicuitry, (not shown). Note that detectionmodule 4 is protected from direct illumination by the Primary beam by abeam stop 18, or a beam dump. Detection module 4 produces an electricalsignal caused by the scatter from the particle passing through thePrimary beam. Detection module 5 produces an electrical signal caused bythe scatter from the particle passing through the Secondary beam.Further electronic circuitry establishes that the ‘valid trajectory’condition applies, ie: that the particle has indeed passed through themeasurement space. Once this has been established, the magnitudes of thesignals from all detection modules are transferred to an electronicprocessing circuit where an assessment of particle shape and orientationcan be made.

FIG. 4 illustrates the derivation of the scatter signals from differentparticle shapes, a spherical droplet, a columnar particle, and aflake-like particle. The top part of FIG. 4 shows the areas covered bythe detector modules 6 to 11 in their hexagonal arrangement superimposedon the typical scattered light patterns from the three types ofparticle. The graphs underneath each of these examples shows the outputsthat would be expected from the detector modules, illustrating how aspherical droplet would yield equal outputs for all six detector moduleswhilst a columnar particle would yield typically two high and four loweroutputs, and a flake would yield a single predominant output.

The nature of the processing of the data from the detector modules isillustrated in FIG. 5. The scattered light intensity signal magnitudesfrom detector 4 and detectors 6 to 11 inclusive are fed into a patternclassification processor. This processor is designed to recognizeparticular patterns of data and to ascribe the corresponding particle toan appropriate class. The simplest class is that of spherical particles,for which the signals from detectors 6 to 11 should be equal to withininstrument measurement accuracy. Other particles, such as elongatedcrystals, will produce typically two high values in the outputs 6 to 11,with the rest being low. In every case, the signal magnitude fromdetector 4 is used as a measure of particle size, larger particlesgenerating proportionally higher values. The output of the patternclassification processor may be of the form of a series of sizedistributions as shown, with spherical particles being one class ofparticle and other particle shapes (such as column or fibres) beinganother. There can be as many classes of particle shape as is desired.

The invention may be applied to the measurement and characterization ofparticles, typically in the range 0.3 μm to 100 μm in size, carried in afluid medium, either gaseous or liquid wherever the fluid medium isflowing in a direction orthogonal to the plane of the measurement spacedefined by the intersecting laser beams. The fluid may be travellingalong a pipe or tube as may be found in many industrial plantsituations, water processing works, etc. In such a case the opticalelements of the invention would reside outside the pipe or tube withoptical access to the fluid via suitably placed windows in the tubewalls.

However, a specific embodiment of the invention for use as anaircraft-mounted detector for atmospheric particles is shownschematically in FIG. 6. The detector modules 4, 5, 6, 7 and 8 can beseen supported in a rigid mounting assembly 19. The motion of theaircraft is from left to right as arrowed such that particle laden air(which remains essentially stationary relative to the earth) passesthrough the intake 20 and out through the vent 21. The internal diameterof the intake is typically 30 mm, sufficient to ensure that particlesnear the axis of the intake and which will subsequently pass through themeasurement space of the instrument are unaffected by the presence ofthe instrument until after their measurement has taken place. Protectingthe optical assembly from the external environment is a shroud 22.Behind the bulkhead 23 are mounted the two lasers 24, together with thedata acquisition electronics, the pattern classification processor, andrequired power supplies. A preferred embodiment of the patternclassification processor is a Radial Basis Function neural network. TheRadial Basis Function is arguably one of the simplest forms ofartificial neural network, is well documented in pattern classificationtexts, and may be considered to be prior art. Data from the instrumentis fed to an inboard computer via communication data lines carriedthrough the aircraft wing, thus providing the aircraft flight crew withvirtually instantaneous data relating to the nature of the particulatecloud through which the aircraft is flying.

What is claimed is:
 1. A particle analyzer for use in characterizing theshape of a particle in a fluid medium, said particle analyzercomprising: means for generating a first beam of radiation, said firstbeam having a first wavelength; means for generating a second beam ofradiation, said second beam having a second wavelength, said first andsecond wavelengths being different, said first and second beamsintersect each other, said intersection comprising a virtual measurementspace, said virtual measurement space located within a flow of saidfluid medium and said particle passes through said virtual measurementspace without being affected by said flow; first detection array ofdetectors for detecting radiation of said first wavelength scattered bysaid particle passing through said virtual measurement space, eachdetector providing a first radiation signal responsive to said detectedfirst wavelength radiation at said each detector; at least one seconddetector adapted and arranged to detect said second wavelength radiationscattered by said particle passing through said virtual measurementspace and provide a second signal responsive to said detected secondwavelength radiation; and means for comparing the first and secondradiation signals to exclude first radiation signals which are nottimewise coincident with second radiation signals and thereby indicatingsaid particle in said virtual measurement space and for comparing saidfirst radiation signals with data from particles of known shape toprovide a shape indicative signal for said particle.
 2. A particleanalyzer as claimed in claim 1 and wherein the first radiation beam hasa cross-section which is substantially a narrow ellipse.
 3. A particleanalyzer as claimed in claim 1 and wherein the second radiation beam hasa cross-section which is substantially circular.
 4. A particle analyzeras claimed in claim 3 and wherein the diameter of the second radiationbeam is less than the widest breadth of the first radiation beam.
 5. Aparticle analyzer as claimed in claim 1 and wherein the first and secondbeams intersect at an angle of substantially 60°.
 6. A particle analyzeras claimed in claim 1 and wherein each of the first and second detectionmeans comprise a lens array, a photodetector and respectively a firstand a second optical wavelength filter which allows only light from thefirst or second radiation beam to reach the respective photodetectors.7. A particle analyzer as claimed in claim 6 and having means definingan aperture in front of the detectors adapted to restrict the field ofview thereof to substantially the virtual measurement space.
 8. Aparticle analyzer as claimed in claim 1, wherein the means for comparingprovides an output indicative of particle size.
 9. A particle analyzeras claimed in claim 1 and adapted for distinguishing between a particleof ice crystal and a particle comprised of a super-cooled water droplet.10. A particle analyzer as claimed in claim 1 and adapted for deploymenton an aircraft.
 11. A particle analyzer as claimed in claim 10 andadapted and arranged to provide an instantaneous indication to anaircrewman of the nature of particles in the vicinity of the aircraft.12. A method of particle analysis for analyzing at least one of theshape and size of a particle, said method comprising the steps of:directing a first beam of radiation having a first wavelength at asecond beam of radiation having a second wavelength, the two wavelengthsbeing different, the intersection of the two beams form a virtualmeasurement space through which said particle passes; detecting with anarray of detectors at a plurality of locations radiation of said firstwavelengths scattered by said particle and generating first radiationsignals; detecting with a second detection means radiation of saidsecond wavelength scattered by said particle and generating electricalsignals corresponding thereto, and processing said first and secondradiation signals for coincidence indicating that radiation wasscattered from a particle located in said virtual measurement space,and, analyzing and comparing said first radiation signals with data fromparticles of known shape to derive at least one of shape and size datarelating to said particle.
 13. A method as claimed in claim 12 andadapted for discriminating between water droplets and ice crystals.